Although the design of structures under normal loading conditions aims at meeting serviceability and ultimate strength requirements by providing strength, stiffness and stability, it has been recognized recently that to effectively and safely resist extreme loading conditions such as earthquakes and blast loads, a fundamentally different approach must be used. It is economically unfeasible as well as being potentially unsafe to design structures for linear elastic response under such loading conditions, especially if, as a result of this design philosophy, no ductility capacity is provided in the system. This implies that the nonlinear behavior of yielding systems, which limits the seismic forces induced in structures, is a highly desirable feature.
For yielding systems, the energy dissipated per cycle through hysteretic yielding (inelastic deformations) is generally associated with structural damage. Such yielding systems are expected to sustain residual deformations which can greatly impair the structure and increase repair costs. This raises important questions which usually remain unanswered following extreme loading conditions: does a structure that has undergone a certain level of inelastic deformation still provide the same level of protection as before? Must all yielded elements be replaced? Must the state of the material at every location where yielding has taken place be assessed?
There also exists a strong belief, mainly from the public, that a structure designed according to the latest seismic codes, for example, would require little or no structural repair and would result in minimal disruption time following an earthquake. Current research efforts in earthquake engineering still embrace this philosophy of achieving stable hysteretic response of predetermined elements of the structure. Structural damage and residual deformations are therefore expected under design level earthquakes.
For example, traditional steel braced frames are designed primarily to assure life safety under a major earthquake. They are expected to sustain significant damage after an earthquake due to repeated cycles of brace tension yielding and brace compression buckling. Furthermore, as a direct consequence of the damage induced in these elements, the final state of the entire building is likely to be out of plumb. Similar response is also expected from the other conventional steel, reinforced concrete, masonry and timber structural systems (moment-resisting frames, walls, etc.). Poor structural performance also results in damage to operational and functional components of buildings, such as architectural components, building services or building contents. Both structural and non structural damage can impact on the safety and rescue of building occupants and can lead to interruption of building operations.
This reality has important consequences as to the costs of repair and the costs induced by disruption time following an important earthquake. Note that a structure that is found to be structurally sound after an earthquake may be condemned if the costs of straightening are elevated or if it appears unsafe to occupants. Increasingly, owners of structures in seismic prone areas that are faced with the expected state of their structure following a major earthquake often opt to directly implement higher performance systems. Furthermore, insurance companies are also increasingly basing their premiums on expected damage costs, and with this additional incentive, the number of owners that will adopt high performance systems for new or existing structures is likely to increase.
The current state-of-the-art for specialized dampers that are used to improve seismic performance mainly consists of either hysteretic (yielding), friction, viscously damped, viscoelastic systems or shape memory alloys. The hysteretic (yielding) systems consist of elements that are designed to undergo repeated inelastic deformations and that exhibit variable hysteretic responses.
A first family of such systems is referred to as yielding systems such as the buckling restrained braces or yielding steel plates. Yielding systems have been successfully implemented in numerous projects in Asia and North America. A second family of such systems is referred to as friction systems, of which one of the most popular is the Pall system. This system has been implemented in a very large number of structures in the past 15 years.
Note that none of these two families of systems exhibits self-centering properties, which can negatively impact on the overall performance of structures when subjected to earthquakes and other severe or extreme loads and may result in permanent deformations.
Viscous systems are specialized devices that exhibit a velocity dependent force and increase the damping of the structure thus reducing the response under seismic loading. Viscoelastic dampers also exhibit a velocity dependant force to increase damping while providing an additional elastic restoring force in parallel. Structures equipped with viscous and visco-elastic dampers require the main structural system to provide sufficient elastic stiffness and strength to resist the applied loads. These devices do not assure self-centering properties if the main structural elements undergo inelastic deformations.
A shape memory alloy is generally a metal that regains by itself its original geometrical configuration after being deformed or heated to a specific temperature. Shape memory alloys generally provide highly specialized production capability, but are generally expensive materials.
To date, self-centering behavior has mainly been achieved by specialized dampers comprised of complex inter-connected spring elements that require sophisticated fabrication processes and shape memory alloy materials that are prohibitive in most common structural projects because of elevated costs.
In U.S. Pat. No. 5,819,484 entitled “Building structure with friction based supplementary damping in its bracing system for dissipating seismic energy” (issued on Oct. 13, 1998), Kar teaches about a brace apparatus that provides re-centering capabilities through a friction spring energy dissipating unit, but which converts tension and compression applied to the apparatus into compression exerted on the stack between the two ends of the apparatus which are mountable to two portions of a building.
In U.S. Pat. No. 5,842,312 entitled “Hysteritic damping apparati and methods” (issued on Dec. 1st, 1998), Krumme et al. teach about damping apparatus using one or more tension elements fabricated from shape-memory alloy to provide energy dissipation. However, the apparatus of Krumme et al. which has two relatively moving bracing members linked together by the tension elements provides that some tension elements are involved during a force loading, but the self-centering behavior of the damping apparatus results from specific nonlinear material properties and do not involve mechanical interaction between elastic components.
The previous discussion leads to suggest that an optimal extreme load resistant system should:
i) incorporate the nonlinear characteristics of yielding structures to limit the forces imposed on the system by the severe or extreme loading, and dissipate input energy to control deformation;
ii) reduce the cost of repairs of the structure by encompassing re-centering properties allowing it to return to its original position after the extreme loading;
iii) further reduce the cost of repair by minimizing the occurrences of damages to the main structural elements.
Optimal resistance to severe or extreme loading increases the performance level of structures in the event of a major earthquake, hurricane or the like which sometimes occur in highly populated urban areas. Structures equipped with these high performance elements significantly offer better responses to such extreme loading with minimal damage, reduced repair costs and disruption time.
Furthermore, these systems may be very attractive to local, provincial and federal government facilities as well as to owners and managers of critical facilities that must remain functional during and immediately after major or catastrophic events.